Field of the Invention
The present invention relates generally to a DC power converter for producing an output voltage different from the input voltage to the converter, and more particularly to an inductive DC power converter which optimizes the power conversion from a variable DC voltage input to a variable DC voltage output while both providing maximum efficiency and power from the inductor during conversion and avoiding saturation of the inductor during conversion.
DC power converters have the basic requirement of supplying a particular output DC voltage level from a typically constant input DC voltage level. Unlike the less sophisticated AC transformer, which is relatively basic, the DC power converter must operate using a relatively constant level of input voltage. Two techniques which have been developed to convert a DC input voltage to a desired DC output voltage use reactive circuit elements, typically capacitors and inductors, as energy storage media. Such devices operate by periodically switching the reactive element into alternating connection with the DC input voltage to charge the reactive element, and with the DC output circuit to discharge the reactive element.
Due to the nature of capacitors and inductors, inductors have found greater favor for use in DC power converters than have capacitors. This is largely because inductors capable of providing a specific capacity are cheaper, smaller, and easier to work with than are capacitors having the same capacity (with the specific exception of device having only very small capacities). It has been recognized for some time that inductors may advantageously be used as energy storage media, which may be switched between an input voltage and an output circuit to convert DC power.
Early examples of inductive power converters may be found primarily in welding circuits, such as for example in U.S. Pat. No. 2,276,796, to Rogers, and in U.S. Pat. No. 2,276,851, to Livingston. The Rogers device uses a shunt resistor and a relay to switch the inductor between the input and the output when a sufficient current is detected to exist in the inductor. The Livingston device triggers on voltage rather than on current, and utilizes at the monitored voltage a waveform derived from an unfiltered, full wave rectified sine wave. These devices are used to generate a welding current, and are not very precise due to the nature of a welder type device.
More sophisticated devices using an inductive coil have been developed, such as the simple solenoid, which uses the magnetic field generated by an inductor to drive a plunger to perform mechanical work. Examples of such devices are found in U.S. Pat. No. 4,173,030, to Rabe, and in U.S. Pat. No. 4,293,888, to McCarty. The Rabe device, which uses the solenoid to actuate a fuel injector, times the application of voltage to the solenoid coil depending on the level of voltage, with the desired effect being to consistently achieve operation of the fuel injector. The McCarty device, which is a print hammer drive circuit, measures for a minimum current required to consistently actuate the print hammer, and applies that current for a brief time period. Neither of these device convert DC power to DC power, both converting electrical energy to mechanical energy.
Recent examples of DC to DC power converters for more sophisticated applications are found in U.S. Pat. No. 3,191,074, to Carruthers et al., and in U. S. Pat. No. 4,511,829, to Wisniewski. The Carruthers et al. device is used to store particularly high levels of energy which may be inductively switched, and it, like the Rogers and McCarty devices, measures current flowing through the inductor to control the operation of the device. The Wisniewski device is more sophisticated, and it uses timed charging with the discharge time being based on the level of current in the output circuit.
It is particularly notable that except for the Rabe device, none of the devices discussed above are designed to be operable with a variable DC input voltage. The Rabe device may also be dismissed since it is not a power converter, but rather a solenoid. It is therefore an object of the present invention to design a DC to DC inductive power converter which is capable of operating with a variable DC input voltage, preferably over a relatively wide range of DC input voltages. This will allow the power converter of the present invention to operate successfully with a broad range of input voltages.
It is also desirable to design a DC to DC inductive power converter which will be capable of supplying an output voltage which may vary widely. Previously known devices may have operated at variable voltage DC outputs, but they were not designed to do so efficiently over a wide range of output voltages. The present invention shall be capable of supplying a wide range of output voltages efficiently, and shall in an alternate embodiment also be capable of simultaneously supplying power at more than one voltage level.
It is also an objective of the present invention to operate in as efficient a manner as is possible. Efficiency of a DC to DC converter has two components, namely input efficiency and output efficiency. Input efficiency is maximized by charging the inductor to a point near saturation, but slightly short of saturation. Saturating the inductor is bad per se from an efficiency standpoint, and it may also saturate solid state electronic switches used to switch the inductor between the input and output circuits. Shortening the charging time significantly beyond the extent necessary to avoid saturation destroys power capability of the device. It is a further object of the present invention to enable charging of the inductor to a point near, but short of, saturation at any or all of the DC input voltages at which the device may be operated.
Output efficiency is dependent on just barely discharging the inductor, but not continuing to attempt to discharge it once it is completely discharged. The failure to just discharge the inductor will result in boosting currents during the next charge cycle, since the current remaining will be added to by additional current during the next charge cycle. This will likely result in saturation, and all of the drawbacks and inefficiencies discussed above. Lengthening the discharge time beyond the time necessary to discharge the inductor damages power capability of the system. It is therefore an object of the present invention to enable discharging of the inductor completely, without wasting any additional time and adversely affecting efficiency.
By simultaneously maximizing efficiency on both the input side of the inductor and the output side of the inductor, overall efficiency and power capability of the device will be maximized. It is a further objective of the device that the beneficial effects previously described be achieved at a reasonable economic cost, and that the device of the present invention not be unduly complex. By achieving these objectives, the system of the present invention will be able to convert a variable DC input voltage into a variable DC output voltage, while maximizing overall power capability of the system unlike any previously known device. It is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.